@@The polymer films on solid supports have numerous technological and biological applications. Instability phenomenon in the ultrathin polymer films on solid substrate is an important factor to influence its technological applications. The dewetting of a single layer of liquid film on the solid substrate is reasonably well understood. However, the dewetting behavior of two immiscible layers of thin films on a solid substrate is more complicated and interesting. Bilayer films and multilayer films become more and more important in the technological applications and the fundamental scientific interests. Recently, many papers have reported the dewetting process of polymer bilayers by experiments, simulations and theories, and have shown a richer dynamics and dewetting pathways. The evolution of polymer bilayers is driven by the effective molecular interactions between all the three interfaces separating the four media: substrate, the lower layer, the upper layer, and surrounding gas (air). Many experiments have reported that only the upper layer can dewet on the liquid substrate and only the lower layer dewets between the upper layer and the solid substrate. However, few experiments reported the richer dewetting behavior of some bilayer systems, for example, both two layer are unstable. The study of these bilayer systems is necessary in order to further understand the dewetting of the bilayer systems. The previous paper has presented a stability phase diagram of the dewetted structures for a PS/PMMA bilayer by changing the ratio of the two film thickness. It is well known that the liquid-liquid interfacial tension may be an important factor to influence the dewetting behavior of the bilayer system. The change of the molecular weight of polymer and the annealing temperature can strongly influence the liquid-liquid interfacial tension. In the paper, we will investigate the influence of the molecular weight of polymer and the annealing temperature on the dewetting pathways of the bilayer system and the liquid-liquid interfacial tension. In our experiments, we investigate the dewetting behavior of the bilayer of air/PS/PMMA/the silanized Si wafer and find the two competing dewetting pathways in the dewetting process. The upper layer dewets on the lower layer (the dewetting pathway 1, the liquid-liquid dewetting) and the bilayer film rupture on the solid substrate (the dewetting pathway 2, the liquid-solid dewetting). The previous studies did not indicate that the two dewetting pathways coexist in the dewetting process. However, our experiments suggest that the two dewetting pathways may coexist in the dewetting process of the bilayer system. The dewetting pathway 1 occurs on the liquid substrate; however, the dewetting pathway 2 occurs on the solid substrate. We change the molecule weight of polymer, film thickness and the annealing temperature to study the competing relation between the liquid-liquid dewetting pathway and the liquid-solid dewetting pathway.
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